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Abstract: Fracture Networks, Fracture Connectivity, and Environmental Implications

Gregory C. Ohlmacher

Detailed outcrop-scale mapping has revealed fracture networks (set of fractures), which result from combinations of tectonic stresses, stress relief, and blasting. Because of the intersecting nature of the fractures associated with these sets, they form potential pathways for fluid migration, including groundwater. Other factors, however, also affect the fractures' ability to transmit fluids. These include aperture, fillings, roughness, density, and connectivity. Analysis of fracture networks for environmental purposes is primarily done using statistical methods that provide most of the data required for fracture permeability models. However, missing from this data is the actual connectivity between the fractures. This can be obtained from detailed outcrop-scale structure sections. A racture set is composed of a number of fracture segments that may or may not link to form conduits. This segmentation may alter the ability of what would otherwise be a connected fracture network to convey fluids. Thus, this lack of connectivity would decrease the fracture permeability, which might reduce the water supply potential, but improve the potential for waste storage. The examination of the connectivity of vein and joint networks mapped in detail reveals some networks have at least two sets of well-connected intersecting fractures forming a two-dimensional network for fluid flow, while other networks have one well-connected set and a secondary set of poorly connected fractures. This latter case would have good permeability in one direction and poor in the second.

AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994